This proposal aims to define the role of the tumor suppressor p53 in the regulation of metabolism, as well as the consequences of the loss of this function in tumor pathogenesis. p53 holds the distinction of being the most frequently mutated gene in human cancers, and its inactivation is essential not only for the formation of a remarkably wide range of tumors, but also for their continued survival and proliferation. Being able to understand the mechanism by which p53 suppresses tumorigenesis has been a central objective in cancer biology, one which has important implications in the treatment of a plethora of cancer types. Among the cellular response elicited by p53, emerging evidence has indicated that metabolic modulation and senescence are crucial for tumor suppression. Tumor cells rely on re-programmed metabolism to rapidly accumulate biomass and to effectively minimize oxidative damages. This proposal focuses on malic enzyme and isocitrate dehydrogenase (IDH). These are major enzymes that generate the reducing equivalent NADPH, which is essential for biosynthesis and anti-oxidant defense. They are also associated with the tricarboxylic acid cycle (TCA cycle), the central metabolic hub, and likely play critical roles in he metabolism of glucose and especially glutamine, two major nutrients for tumor cells. In our preliminary studies, we found previously unanticipated mutual regulation between p53 and malic enzymes. p53 suppresses the expression of malic enzymes, while down-regulation of malic enzymes reciprocally activates p53 and modulates the outcome of p53 activation, leading to senescence. Furthermore, we found that p53 suppresses the expression of IDH1, a major IDH isoform, in unstressed and especially stressed cells. These findings suggest that p53 may function as both a central sentinel and a master regulator of NADPH metabolism, linking the metabolic state of the cell with the cell fate decision. We plan to investigate the dynamic interplays between p53 and malic enzymes/IDH1. Our central hypothesis is that mutual regulation between malic enzymes and p53, as well as the suppression of IDH1 by p53, modulates biosynthesis and anti-oxidant response, and contributes to p53-mediated tumor suppression. We propose three specific aims: (1) Determine the functions of p53 and malic enzymes in glutamine metabolism; (2) Elucidate the role of p53 in regulating IDH1 and IDH1-assoicated metabolic fluxes; and (3) Define the role of malic enzymes in regulating p53 activation, anti-oxidant response, and senescence. The proposed studies will improve our understanding of key aspects of metabolic regulation and their link to p53-mediated cell fate decision, and may provide a rationale for targeting these NADPH-generating enzymes as a new therapy for cancer.